Photochromic glasses containing, in their compositions, silver and one or more halogens are well known. The term "halogens" is used here to include only chlorine, bromine and iodine. Such photochromic glasses were first described in U.S. Pat. No. 3,208,860 (Armistead et al.). As there described, the glass compositions, in addition to silver and halogens, consist essentially by weight on the oxide basis of 40-76% SiO.sub.2, 4-30% B.sub.2 O.sub.3, 4-26% Al.sub.2 O.sub.3 and R.sub.2 O selected from 2-8% Li.sub.2 O, 4-15% Na.sub.2 O, 6-20% K.sub.2 O, 8-25% Rb.sub.2 O and 10-30% Cs.sub.2 O, these essentials constituting at least 85 % of the glass composition.
A second generation of silver halide-containing, photochromic glasses is described in U.S. Pat. No. 4,190,451 (Hares et al.). The compositions of these glasses are described as consisting essentially of 0-2.5% Li.sub.2 O, 0-9% Na.sub.2 O, 0-17% K.sub.2 O, 0-6% Cs.sub.2 O, 8-20% total R.sub.2 O, 14-23% B.sub.2 O.sub.3, 5-25% Al.sub.2 O.sub.3, 0-25% P.sub.2 O.sub.5, 20-65% SiO.sub.2, 0.004-0.02% CuO, 0.15-0.3% Ag, 0.1-0.25% Cl and 0.1-0.2% Br, RO-free other than CuO, and the weight ratio of Ag:(Cl+Br) is 0.65-0.95.
Photochromic glasses based on silver halide crystals are produced by dissolving silver and halogens in a suitable base glass melt. Upon a controlled thermal treatment, a silver halide crystal phase is precipitated in the glass. These crystals are subject to photolysis, that is, separation under the influence of short wave radiation such as contained in sunlight, and recombination upon removal of the radiation. This gives rise to photochromism, that is, reversible darkening behavior. The glass is essentially colorless in the faded, or undarkened, state.
It is possible to impart permanent coloration to photochromic glasses by adding conventional glass colorants to the glass batch in minor amounts. Such known colorants include oxides of the transition metal oxides, nickel, cobalt, and chromium. This practice of imparting a permanent tint to photochromic glasses is described in the Hares et al. patent noted above.
More recently, another method of inducing permanent coloration in a photochromic glass has been described. This method involves thermal treatment of the glass in a reducing atmosphere. This involves at least partially reducing the silver to impart the permanent coloration. A flowing stream of hydrogen is commonly prescribed as the reducing atmosphere. The method is described in detail, for example, in U.S. Pat. No. 4,240,836 (Borrelli et al.).
Difficulties have been encountered in using glass colorants, such as the transition metal oxides, to permanently tint otherwise transparent photochromic glasses. In particular, in some glasses the cosmetic effects obtained using such chromophores tend to have an undesirable greenish cast.
The visible absorption characteristics imparted by the chromophores of these oxides are influenced by the glass matrix. Likewise, photochromic performance depends on the glass matrix. Consequently, commonly known techniques for adjusting transmission characteristics are limited.
There are two recognized methods for decolorizing a greenish color such as that induced by iron oxide in glass. One method involves adding an oxide of manganese. The other involves adding the oxide of the rare earth erbium. The addition of manganese is ruled out because, in amounts adequate for the purpose, it has an adverse effect on photochromic performance. The cost, and the uncertain availability, of erbium oxide of adequate purity is such as to make that method undesirable.
It is a purpose of the present invention to provide a viable alternative to these known methods. A further purpose is to provide an effective method of neutralizing the greenish cast encountered in some photochromic glasses permanently tinted by chromophores of transition metal oxides. Another purpose is to provide a means of shifting the chromaticity point of a permanently tinted, photochromic glass to neutralize the greenish cast of the glass color. A particular purpose is to provide photochromic sunglasses having a more pleasing appearance for the wearer.